Dietary restriction (DR) is the only intervention known to prolong lifespan and reduce the rate, of aging in animal species. The question of whether DR may give similar benefits in humans remains unknown and will be the subject of this proposed study. Many humans impose voluntary dietary energy restriction on themselves, and psychometric measures of the extent of this "dietary restraint" will be the basis of a study of DR in humans. In particular, we will be investigating the effects of DR on age-sensitive metabolic parameters that are independent of body size and by doing this will determine whether DR has health benefits for women over and above any benefit gained as a result of weight loss. The primary hypotheses to be tested are: l) psychometrically-measured dietary restraint is associated with significant reductions in total energy expenditure and energy intake in middle-aged women, as previously reported in young women, independent of level of physical activity and body composition; 2) dietary restraint, in combination with consumption of adequate intakes of protein, vitamins and minerals and independent of level of physical activity and body composition, is associated with alterations in a range of selected age-sensitive metabolic and physiologic variables, suggesting a reduced rate of biological aging in middle-aged women. The subjects will be 66 healthy post-menopausal women aged 55-62 years, with half classified as long-term restrained eaters and half classified as long- term unrestrained eaters. Group matching will be performed for body composition, level of physical activity, ancestral lifespan and other factors influencing morbidity and mortality. In an all day study, determinations will be made of usual nutrient intakes, free-living total energy expenditure, resting metabolic rate, body composition (body fat, lean body mass, total bone mineral, and fat distribution), immune function (in vivo delayed-type hypersensitivity, in vitro T-cell mitogen stimulation of lymphocytes, and cytokine and prostaglandin E2 production by lymphocytes), insulin resistance, circulating insulin-like growth factor 1, thyroid status, blood lipid profile (total cholesterol, HDL, LDL, VLDL and triglycerides), and cardiopulmonary function (maximum oxygen consumption, blood pressure, forced vital capacity and forced expiratory volume in l second). The effects of dietary restraint will be evaluated by comparing results in the two groups using standard statistical techniques. We anticipate that the results of this investigation may have far-reaching consequences with regard to the role of DR in maintaining long-term health in the adult population.